1. Field of the Invention
The invention relates generally to high intensity discharge (HID) lamps and ballasts, and more particularly to a very low cost (VLC) ballast which avoids the need for a separate ramp generator circuit to generate a ramp signal to modulate the average lamp power and achieve desired FM modulation for lamp stability.
2. Description of Related Art
Operation of high intensity discharge (HID) lamps, e.g., metal halide lamps, by electronic ballast results in a phenomena (problem) referred to as acoustic resonance. Acoustic resonance causes the arc within the lamp to girate, flicker, and even extinguish when a ballast circuit operates the HID lamp at frequencies greater than a few kilohertz. High frequency operation in HID lamps is most desirable but for the acoustic resonance problem.
The arc tubes employed in HID lamps are hollow tubes of alumina, quartz or hard glass shaped in various sizes with cupped or conical ends filled with gas at several different pressures, depending on the operating state of the lamp. The pressure increases within the lamp as the lamp heats up. What are referred to as xe2x80x9corgan pipexe2x80x9d (acoustic) resonances can occur at different lamp operating frequencies depending on the size, shape, and pressure within the arc tube. Operation at or near a resonance point (that is, at a particular high frequency) will result in acoustic resonance, i.e., arc flicker, arc stretch, arc elongation and spiraling, and even extinguishing the arc and causing arc tube breakage.
Various attempts are known to overcome the effect of acoustic resonances at high frequency. For example, U.S. Pat. No. 4,373,146, to Bonazoli, et al., discloses a method and circuit for operating a high intensity discharge (HID) lamp in a mode which minimizes or avoids acoustic resonance inside the arc tube. To do so, Bonazoli frequency modulates a carrier waveform in the kilohertz range to provide a variable frequency AC output. By varying the frequency of the ac signal, the lamp is not driven at any particular frequency for a substantial length of time, e.g., on the order of milliseconds. This FM output is provided across the electrodes of an HID lamp to drive the lamp while minimizing or avoiding acoustic resonance.
The Bonazoli circuit, however, is not low cost. The Bonazoli circuit is reproduced in prior art FIG. 1. The Bonazoli circuit includes a DC source 2 with an output connected to an inverter 6, the inverter DC input coupled to the output of the DC source 2. A carrier waveform (square wave) generator 8 in the kHz range is included which has an output coupled into inverter 6, as well as ramp generator means 20 (a conventional ramp generating circuit) for frequency modulating the carrier waveform to provide the inverter 6 with a variable frequency signal. A starter means 4 for coupling the AC signal across the lamp electrodes 12, 14 is also included. The Bonazoli square wave generator 8 operates at a voltage-controlled frequency of at least 20 kHz modulated by the sawtooth signal generated by the ramp generator.
During operation, a ramp period could be in a range of 1 to 10 milliseconds (100-1000 Hz.) with a flyback of about microsecond. The application of the ramp voltage to the square wave generator causes the carrier waveform to be swept from 20 to 30 kHz. And as mentioned above, such a mode of operation, because of the constantly changing high frequency, avoids resonance. A problem with the Bonazoli circuit resides in the fact that Bonazoli and like apparatus and methods for dealing with the problem of acoustic resonance within HID lamps is that such a circuit must incur the cost of a ramp generator circuit to provide a ramp signal to modulate the carrier waveform (FM) to drive the lamp.
It is therefore an object of the present invention to provide a simple and eloquent circuit for driving an HID lamp, e.g., a metal halide lamp, with a frequency modulated square wave which avoids the shortcomings of the ballast circuits of the prior art.
It is another object of the invention to provide, in a VLC ballast where FM is used for feedforward control to conform to line regulation, a simple R-C circuit is utilized to couple the low frequency (100-120 Hz.) ripple derived from a buffer capacitor into the frequency control portion of the ballast circuit (e.g., to vary the frequency of a signal output by a VCO). Utilizing the inherent AC ripple signal avoids the need for a separate and additional ramp generator circuit to generate a ramp signal to frequency modulate the average lamp power signal. The invention is particularly useful in lamps having a long aspect ratio, that is, at least one (1) and preferably greater than three (3).
To that end, a first embodiment of the present invention is a very low cost (VLC) ballast for driving HID lamps. The VLC ballast comprises an AC to DC converter block which includes a buffer capacitor (CBUF) for receiving AC from a main source of AC power and converting it to DC. The output of the AC to DC converter provides to an HF inverter and ballasting element. The AC signal component captured by the buffer capacitor is filtered by a frequency control circuit constructed and provided to a high voltage driver to modulate the carrier waveform provided by the HF inverter and ballasting element to drive the lamp. The frequency control circuit includes a capacitor in parallel with a first resistor, the parallel combination in series with a second resistor. The resulting sawtooth-like signal derived from CBUF (as shown in FIG. 4A) is summed with the carrier signal thereby modulating same. Since the AC is either 50 or 60 Hz, the sawtooth will have a frequency of about 100 or 120 Hz. If the carrier waveform is around 80 kHz, for example, the driver will modulate the 80 kHz carrier with the filtered xe2x80x9csawtoothxe2x80x9d signal, to vary the frequency of the lamp driving signal from around 75 to around 85 kHz.
The reader must note that the frequency range of HID lamps is conditioned upon power outage of the HID. For example, the HID lamp of the first embodiment is about 39 Watts. In a case of a 70 Watt lamp, the range would vary from 45-55 kHz, and a lamp putting out about 150 Watts would sweep from about 18 to about 26 kHz. The AC component of the buffer capacitor is used to modulate the average lamp power to achieve the desired FM modulation for stable operation of the lamp, that is, unnoticeable acoustic resonance.
The present invention also includes a very low cost method for operating a high intensity discharge (HID) lamp. The method includes receiving AC and converting it to DC in an AC-to-DC converter. The DC is then provided to an HF inverter and ballasting circuit, wherein a carrier signal for driving an HID lamp is generated. A ripple from atop the DC generated in the AC-to-DC converter is captured by a buffer capacitor and provided as an input to a driver to drive the HF inverter and ballasting circuit. The sawtooth or ripple modulates the carrier frequency (lamp average power) which is provided across the HID lamp electrodes such that a swept frequency signal is generated. Hence, the lamp is driven by a frequency modulated signal which minimizes the effects of acoustic resonance on the HID lamp.